1. Field of the Invention
The present invention relates in general to steering angle sensors for detecting the steering angle of a steering wheel of wheeled motor vehicles, and more particularly to the steering angle sensors of a type that is employed in an electric power steering device for detecting the steering angle of the steering wheel.
2. Description of Related Art
Hitherto, various types of steering angle sensors have been proposed and put into practical use particularly in the field of is wheeled motor vehicles. One of them is disclosed in Japanese Translation of PCT International Application 2001-505667, which generally comprises a first gear that is fixed to a steering shaft to rotate together therewith, a second gear that is meshed with the first gear and has a magnet and a third gear that is meshed with the first gear and has a magnet, in which the number of teeth of the second gear is different from that of the third gear and the magnets of the second and third gears constitute respective MR sensors. That is, based on voltage variation of the MR sensors, a steering angle (viz., absolute steering angle) defined between a neutral angular position of the steering wheel and a current angular position of the steering wheel is detected.
However, due to its inherent construction of the steering angle sensor of the above-mentioned Published Application, the detected steering angle tends to have a detection error. That is, in the known steering angle sensor, the first gear is directly connected to the sheering shaft to rotate together therewith. Thus, when, during handling of the steering wheel by a driver, the steering shaft is inclined, the first gear is inevitably inclined following the inclination of the steering shaft. However, due to inclination of the first gear thus made, transmission manner of rotation of the first gear to the second and third gears is considerably affected or deteriorated, which causes generation of the above-mentioned detection error.
Accordingly, it is an object of the present invention to provide a steering angle sensor which is free of the above-mentioned drawback.
That is, according to the present invention, there is provided a steering angle sensor which suppresses generation of the above-mentioned detection error even when the steering shaft is inclined.
In accordance with a first aspect of the present invention, there is provided a steering angle sensor which comprises a steering shaft (6) rotating together with a steering wheel (1); a sensor housing (16) arranged to enclose the steering shaft (6); a first gear (24) rotatably received in the sensor housing (16), the first gear (24) including an annular body portion (24A) disposed about a given part of the steering shaft (6) leaving a given annular clearance (t1) therebetween and an annular toothed portion (24B) formed on an outer periphery of the annular body portion (24A); a second gear (25) rotatably received in the sensor housing (16), the second gear (25) having a magnetic member (29) at a center portion thereof and an annular toothed portion meshed with the annular toothed portion (24B) of the first gear (24); a third gear (26) rotatably received in the sensor housing (16), the third gear (26) having another magnetic member (30) at a center portion thereof and an annular toothed portion meshed with the annular toothed portion of the second gear (25), the numbers of respective teeth of the second and third gears (25, 26) being mutually indivisible to provide a unit of the second and third gears (25, 26) with a given speed reduction ratio; a first MR element (31A) detecting a change of a magnetic field produced by the magnetic member of the second gear (25) for obtaining a first rotation angle of the second gear (25); a second MR element (31B) detecting a change of a magnetic field produced by the magnetic member of the third gear (26) for obtaining a second rotation angle of the third gear (26); an elastic member (23) compressed in the given annular clearance (t1) between the steering shaft (6) and the first gear (24), a thickness of the elastic member (23) in a non-compressed condition being larger than a thickness of the given annular clearance (t1); and a control unit (18) that obtains an absolute steering angle of the steering wheel (1) based on the first and second rotation angles of the second and third gears (25, 26).
In accordance with a second aspect of the present invention, there is provided an electric power steering device which comprises a steering mechanism including a steering shaft (6) that rotates together with a steering wheel (1) and a rack bar (8) that steers steered road wheels of an associated vehicle in response to rotation of the steering shaft (6); an electric motor (19) by which a steering assist force is fed to the steering mechanism; a steering angle sensor that detects an absolute steering angle, the absolute steering angle being a rotation amount of the steering wheel (1) from a neutral angular position of the steering wheel (1) where the steered road wheels face in a straight advancing direction to a current angular position of the steering wheel (1); and a control unit (18) that controls the electric motor (19) based on an output signal from the steering angle sensor, wherein the steering angle sensor comprises a sensor housing (16) arranged to enclose the steering shaft (6); a first gear (24) rotatably received in the sensor housing (16), the first gear (24) including an annular body portion (24A) disposed about a given part of the steering shaft (6) leaving a given annular clearance (t1) therebetween and an annular toothed portion (24B) formed on an outer periphery of the annular body portion (24A); a second gear (25) rotatably received in the sensor housing (16), the second gear (25) having a magnetic member (29) at a center portion thereof and an annular toothed portion meshed with the annular toothed portion (24B) of the first gear (24); a third gear (26) rotatably received in the sensor housing (16), the third gear (26) having another magnetic member (30) at a center portion thereof and an annular toothed portion meshed with the annular toothed portion of the second gear (25), the numbers of the respective teeth of the second and third gears (25, 26) being mutually indivisible to provide a unit of the second and third gears (25, 26) with a given speed reduction ratio; a first MR element (31A) detecting a change of a magnetic field produced by the magnetic member of the second gear (25) for obtaining a first rotation angle of the second gear (25); a second MR element (31B) detecting a change of a magnetic field produced by the magnetic member of the third gear (26) for obtaining a second rotation angle of the third gear (26); and an is elastic member (23) compressed in the given annular clearance (t1) between the steering shaft (6) and the first gear (24), a thickness of the elastic member (23) in a non-compressed condition being larger than a thickness of the given annular clearance (t1); and a control unit (18) that obtains an absolute steering angle of the steering wheel (1) based on the first and second rotation angles of the second and third gears (25, 26).